Several different types of mount assemblies are presently available to isolate vehicle vibrations, such as produced during operation of automobile and truck engines and transmissions. A type of mount commonly used today is the solid rubber block integrally molded to opposed metal bolts. To support a vehicle engine, for example, a number of solid rubber mounts are attached by the bolts between the engine and the vehicle frame. Under these circumstances, the mounts are required to support the engine in the manner of a spring, and dampen vibrations in the manner of a shock absorber element as well. These dual functions often require conflicting operating characteristics, and thus a trade-off of different design considerations. As a result, the mount as designed, generally reflects a compromise, and therefore provides neither optimum load bearing nor damping properties. Thus, although this type of mount is somewhat effective, there is a need for improvement in balancing the conflicting requirements of load support versus damping control.
Recent developments in hydraulic mount technology have led to successful hydraulic elastomeric mounts particularly adapted for engine mounting. An example of such a mount is disclosed in U.S. Pat. No. 4,588,173 to Gold et al, issued May 13, 1986 and entitled "Hydraulic-Elastomeric Mount".
The hydraulic mount assembly of this prior invention includes a reinforced, hollow rubber body that serves as both the load supporting spring and the damping means. A hydraulic cavity, partially formed by the body, is closed by a resilient diaphragm. This cavity is partitioned by a plate into two chambers that are in fluid communication through a relatively large central opening in the plate. The first or primary chamber is formed between the orifice plate and the body. The secondary chamber is formed between the plate and the diaphragm.
A decoupler is positioned in the central opening of the plate and reciprocates and responds to vibrations. The decoupler movement alone accommodates small volume changes in the two chambers. When, for example, the decoupler moves in a direction toward the diaphragm, the volume of the primary chamber increases and the volume of the secondary chamber correspondingly decreases. In this way, for certain small vibratory amplitudes and generally higher frequencies, fluid flow between the chambers is substantially avoided and undesirable hydraulic damping is eliminated. In effect, this freely floating decoupler is a passive tuning device.
In addition to the relatively large central opening, an orifice track with a smaller flow passage is provided extending around the perimeter of the orifice plate. Each end of the track has an opening; one opening communicating with the primary chamber and the other with the secondary chamber. The orifice track therefore provides the hydraulic mount assembly with another passive tuning component, and when combined with the freely floating decoupler provides at least three distinct dynamic operating modes. The particular operating mode is primarily determined by the flow of the fluid between the two chambers.
More specifically, small amplitude vibrating input such as from smooth engine idling or the like, produces no damping due to the action of the decoupler as explained above. Large amplitude vibrating inputs produce high velocity fluid flow through the orifice track, and accordingly a high level of damping force, and desirable smoothing action is obtained. A third or intermediate operational mode of the mount occurs during medium amplitude inputs resulting in lower velocity fluid flow through the orifice track. In each case of switching from one mode to another, a limited amount of fluid can bypass the orifice track by moving around the edges of the decoupler, smoothing the transition between operational modes.
This basic mount design has proved quite successful, and represents a significant advance over the prior art engine mounts, particularly the solid rubber type. More particularly, hydraulic mounts provide a more favorable balance of load supporting and damping control. It should be appreciated, however, that additional improvement in operating characteristics is still possible.
More specifically, many hydraulic mounts having a single decoupler suffer a significant, dynamic rate increase at higher frequencies. As a result, the mounts do not provide the desired vibration and noise isolation for certain small displacements at these higher frequencies. Consequently engine vibration, such as during idling operation, may be transferred to the driver, especially through the steering wheel or other vehicle operating controls, such as the gear selector.
In the past, efforts have been made to address this problem. As a result of those efforts, a number of "active" mounts have been developed wherein vehicle operating conditions are sensed and the performance characteristics of the mount are tuned to provide the desired damping or decoupling at any given time. Examples of such active systems are found in U.S. Pat. No. 4,756,513 entitled "Variable Hydraulic-Elastomeric Mount Assembly" and U.S. Pat. No. 4,789,142 entitled "Electronic Motor Mount With Magnetic Decoupler", both owned by the assignee of the present invention.
While these active systems effectively address the problem, it should be appreciated that they have a relatively complicated structure and incorporate relatively expensive components, such as transducers and a microprocessor controller. To reduce costs and increase reliability, it would be desirable to provide a fully passive system that successfully addresses the problem. A step in that direction is made in the mount assembly disclosed in copending United States application of Gunderson entitled "Hydraulic Mount with Dual Decouplers", Ser. No. 07/600,899 filed 22 OCT. 90, and assigned to the present assignee. However, further engineering refinement toward the ideal passive engine mount exhibiting still further improved load support and damping control is needed. The optimum mount would substantially eliminate the compromises between these two functions and maximize overall operating performance in each.